fix(fs): align symlinked parent traversal

.agents/notes/implemented/architecture/2026-07-02-fs-per-session-cwd.md

@@ -10,13 +10,15 @@ Filesystem resolution used one plugin-load cwd while bash used the session proje
 
 A valid absolute cwd can itself have two apparent parents: when it contains `symlink/..`, filesystem lookup follows the symlink before applying `..`, while `path.resolve()` erases both components lexically. Resolving sandbox policy lexically while launching bash from the raw cwd granted the unrelated lexical parent, denied writes in the real workspace, and let filesystem tools resolve relative paths into the wrong directory.
 
+An ordinary symlink cwd exposes the same distinction when the requested relative path contains `..`: a process traverses from the symlink's physical target, while `path.resolve(cwd, path)` traverses from its lexical spelling. Reads would therefore select a different file than bash or a sandboxed mutation for the same model-supplied path.
+
 ## Decision
 
-Thread the caller's session cwd into path resolution, exactly as `dsh-tool-bash` already does for `workdir`. When the cwd contains a parent segment, resolve it to its native filesystem identity before any lexical join; ordinary cwd spellings stay stable for display. Reuse the resolved sandbox-policy root for mutations and sandboxed bash calls so one call has one workspace identity. The **caller** (the tool) supplies the cwd; the provider does not read a session or agent.
+Thread the caller's session cwd into path resolution, exactly as `dsh-tool-bash` already does for `workdir`. When either the cwd or the requested path contains a parent segment, resolve the cwd to its native filesystem identity before any lexical join; ordinary cwd spellings stay stable for display when no traversal makes their identity observable. Reuse the resolved sandbox-policy root for mutations and sandboxed bash calls so one call has one workspace identity. The **caller** (the tool) supplies the cwd; the provider does not read a session or agent.
 
 - `FileSystem.resolve` accepts `resolve(path: string, opts?: { cwd?: string; signal?: AbortSignal }): Promise<FsTarget>`. `opts.cwd` is the base a RELATIVE `path` resolves against; an absolute `path` ignores it; omitting `opts.cwd` uses the backend's own default. `opts.signal` cancels resolution when the backend performs I/O. The options object keeps both caller-owned resolution controls together without positional growth.
 - `dsh-fs-local.resolve` uses `resolveLocalTarget(opts?.cwd ?? this.config.cwd, path)`. `config.cwd` stays the default for a caller that supplies none (non-ACP / no-session use, and the single-session stdio demo where `process.cwd()` IS the workspace).
-- `dsh-tool-fs`'s `read`/`write`/`edit` derive the session cwd through a shared `sessionCwd(exec)` helper (`exec.agent?.session.header.cwd`, mirroring bash's `resolveWorkdir`) and pass it to `resolve`. The helper uses native realpath semantics when a parent segment could cross a symlink while retaining ordinary spellings; a sandboxed mutation reuses the complete policy's `workspaceRoot`; a non-agent / headerless caller yields `undefined`, so the backend applies its default.
+- `dsh-tool-fs`'s `read`/`write`/`edit` derive the session cwd through a shared `sessionCwd(exec, requestedPath)` helper (`exec.agent?.session.header.cwd`, mirroring bash's `resolveWorkdir`) and pass it to `resolve`. The helper uses native realpath semantics when a parent segment in either value could cross a symlink while retaining ordinary spellings otherwise; a sandboxed mutation reuses the complete policy's `workspaceRoot`; a non-agent / headerless caller yields `undefined`, so the backend applies its default.
 
 ## Alternatives considered
 
@@ -29,7 +31,7 @@ The default lives in ONE place — the provider's `config.cwd`. `sessionCwd` ret
 ## Consequences
 
 - In the ACP demo the fs tools and bash now agree on each session's workspace; an editor can open any project folder and both tool families act on it.
-- A session cwd containing `symlink/..` resolves to the same physical workspace for bash launch, relative filesystem paths, and the sandbox grant; the lexical parent receives no grant.
+- A session cwd containing `symlink/..`, or an ordinary symlink cwd paired with a parent-traversing relative path, resolves from the same physical workspace for bash, filesystem tools, and the sandbox grant; the lexical parent receives no grant.
 - No change to `FsTarget` identity: `targetKey` is still the realpath of the resolved absolute path, so observed-state keying and symlink identity are unaffected — a correct per-session cwd produces the same key bash targets.
 - Backward compatible: every existing `resolve(path)` call (all in tests) keeps working; the new argument is optional.
 - The single-session stdio demo is unaffected: it supplies no session cwd (its agent's session has no `cwd`), so resolution falls back to `config.cwd = process.cwd()`, which is the workspace.